Okay, Junior has an absolutely unbelievable premise. Get over it. Most movies require a certain "willing suspension of disbelief." Possibly this one requires more than others, but if you can make the leap, it's well worth your time. Arnold Schwarzenegger, nobody's idea of a guy with a feminine side, agrees to use his own body for a human trial of a fertility medicine. When the trial is over, the medicine is proven worthy, but Arnie finds himself unwilling to end the pregnancy. The exact reason is never quite given, but since pregnancy makes him a considerably more likable fellow, than the cold, aloof man he was before, I can deal with that.
The shoe in the machine is Emma Thompson, another scientific researcher who is such a klutz it's amazing she's allowed anywhere near expensive scientific equipment. It seems the egg taken for the fertility experiment belonged to her and was taken without her consent, so Arnie is carrying Emma's baby. Complicating everyone's life further is that Danny DeVito (Schwarzenegger's research partner) learns his ex-wife is pregnant after a one-night stand with one of the staff members of the Aerosmith band, and she wants her ex to deliver the baby. Schwarzenegger and the ex get cozy as "girlfriends"--binge eating, comparing belly sizes--while Arnie starts falling for the likable, if dangerous, Thompson.
And Frank Langella, who seems to have found is true calling in life as a villain, makes a great one in this as he decides to get rich and famous by taking over the research project AND Arnie. (Spoiler alert--it ain't gonna happen, Frankie.)
This is a warm, somewhat corny but very fun movie about role reversal and its effect on two men, two women, and two babies. I've watched it probably a dozen times over the last 20 years, and it's still fun. It's also a testament to Danny DeVito's acting ability. Rather like Christopher Reeve's "you'll believe a man can fly," DeVito and Schwarzenegger will make you believe a man can have a baby.